refactor(ingestion/task): emit kb_id as a single string in ProcessChunksForPipeline (#17802)

## Summary
- `ProcessChunksForPipeline` now sets `kb_id` to a plain string instead
of `[]string{kbID}`, removing an index-physical array shape from the
ingestion domain.
- Stored documents are byte-identical: Elasticsearch overrides `kb_id`
with `datasetID` on write, and Infinity's `transformChunkFields` already
accepts a plain string.
- Infinity is intentionally left unchanged — `service/chunk` paths still
feed `kb_id` as `[]string`, and Infinity handles both forms. The
`dataset` artifact merge (`dataset_artifact_service.go`) is out of scope
for this step.
- Unit assertion updated to expect a string.

## Scope / non-goals
This is the smallest first step (T1) of the index-schema leak cleanup
tracked in #17371. It does **not** move the other leaks (`docnm_kwd`,
`create_timestamp_flt`, position ints) to the engine boundary — those
are later steps behind a read-back golden test.

## Test plan
- `go test ./internal/ingestion/task/indexdoc/...` passes.
- The two `task` "Real" integration tests fail identically on a clean
tree (environment lacks real embedding/parsing); they are pre-existing,
unrelated to this change.

🤖 Generated with [CodeBuddy Code](https://cnb.cool/codebuddy)

internal/engine/infinity/chunk_transform_test.go

@@ -0,0 +1,80 @@
+// Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package infinity
+
+import (
+	"reflect"
+	"testing"
+)
+
+// TestTransformChunkFields_IngestionShape is the T0 unit-tier read-back
+// baseline for the Infinity engine (issue #17371). Unlike Elasticsearch, which
+// stores the suffixed field names verbatim, Infinity reverse-maps the
+// ingestion-emitted names to its own base names inside transformChunkFields.
+// This test calls that transform directly (the engine requires a live Infinity
+// instance, so it cannot be exercised via httptest) and asserts the mapping
+// stays stable. It guards T2: when the suffixing is moved out of ingestion into
+// the engine write boundary, transformChunkFields must keep producing the same
+// base-name document.
+//
+// The input mirrors the shape produced by indexdoc.ProcessChunksForPipeline
+// after T1 (kb_id is a plain string).
+func TestTransformChunkFields_IngestionShape(t *testing.T) {
+	chunk := map[string]interface{}{
+		"doc_id":               "doc-1",
+		"id":                   "chunk-1",
+		"kb_id":                "kb-1",
+		"docnm_kwd":            "sample.md",
+		"content_with_weight":  "hello world",
+		"create_timestamp_flt": float64(123.0),
+		"question_kwd":         []interface{}{"q1", "q2"},
+		"important_kwd":        []interface{}{"k1"},
+		"page_num_int":         int(1),
+		"position_int":         int(2),
+	}
+
+	got := transformChunkFields(chunk, nil)
+
+	want := map[string]interface{}{
+		"doc_id":               "doc-1",
+		"id":                   "chunk-1",
+		"kb_id":                "kb-1",
+		"docnm":                "sample.md",
+		"content":              "hello world",
+		"create_timestamp_flt": float64(123.0),
+		"questions":            "q1\nq2",
+		"important_keywords":   "k1",
+		"page_num_int":         int(1),
+		"position_int":         int(2),
+	}
+
+	for k, wv := range want {
+		gv, ok := got[k]
+		if !ok {
+			t.Errorf("transformed doc missing field %q", k)
+			continue
+		}
+		if !reflect.DeepEqual(gv, wv) {
+			t.Errorf("field %q = %#v, want %#v", k, gv, wv)
+		}
+	}
+
+	// The suffixed ingestion-only names must not leak into the transformed doc.
+	for _, leaked := range []string{"docnm_kwd", "content_with_weight", "question_kwd", "important_kwd"} {
+		if _, ok := got[leaked]; ok {
+			t.Errorf("transform leaked ingestion-only field %q into Infinity doc", leaked)
+		}
+	}
+}